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Observations from radio telescopes like this one appear to indicate that two black holes are orbiting each other, 750 million light years from Earth. (Credit: National Radio Astronomy Observatory)

Approximately 750 million light years from Earth lies a gigantic, bulging galaxy with two supermassive black holes at its center. These are among the largest black holes ever found, with a combined mass 15 billion times that of the sun. New research from Stanford University, published today (June 27) in Astrophysical Journal, has used long-term observation to show that one of the black holes seems to be orbiting around the other.

If confirmed, this is the first duo of black holes ever shown to be moving in relation to each other. It is also, potentially, the smallest ever recorded movement of an object across the sky, also known as angular motion.

“If you imagine a snail on the recently discovered Earth-like planet orbiting Proxima Centauri – a bit over four light years away – moving at one centimeter a second, that’s the angular motion we’re resolving here,” said co-author of the paper, Roger W. Romani, professor of physics at Stanford and a member of the Kavli Insititute for Particle Astrophysics and Cosmology. The team also included researchers from the University of New Mexico, the National Radio Observatory and the United States Naval Observatory.

The technical achievements of this measurement alone are reason for celebration. But the researchers also hope this impressive finding will offer insight into how black holes merge, how these mergers affect the evolution of the galaxies around them and ways to find other binary black-hole systems.

Miniscule movement

Over the past 12 years, scientists, led by Greg Taylor, a professor of physics and astronomy at the University of New Mexico, have taken snapshots of the galaxy containing these black holes – called radio galaxy 0402+379 – with a system of ten radio telescopes that stretch from the U.S. Virgin Islands to Hawaii and New Mexico to Alaska. The galaxy was officially discovered back in 1995. In 2006, scientists confirmed it as a supermassive black-hole binary system with an unusual configuration.

“The black holes are at a separation of about seven parsecs, which is the closest together that two supermassive black holes have ever been seen before,” said Karishma Bansal, a graduate student in Taylor’s lab and lead author of the paper.

With this most recent paper, the team reports that one of the black holes moved at a rate of just over one micro-arcsecond per year, an angle about 1 billion times smaller than the smallest thing visible with the naked eye. Based on this movement, the researchers hypothesize that one black hole may be orbiting around the other over a period of 30,000 years.

Two holes in ancient galaxy

Although directly measuring the black hole’s orbital motion may be a first, this is not the only supermassive black-hole binary ever found. Still, the researchers believe that 0402+379 likely has a special history.

“We’ve argued it’s a fossil cluster,” Romani said. “It’s as though several galaxies coalesced to become one giant elliptical galaxy with an enormous halo of X-rays around it.”

Researchers believe that large galaxies often have large black holes at their centers and, if large galaxies combine, their black holes eventually follow suit. It’s possible that the apparent orbit of the black hole in 0402+379 is an intermediary stage in this process.

“For a long time, we’ve been looking into space to try and find a pair of these supermassive black holes orbiting as a result of two galaxies merging,” Taylor said. “Even though we’ve theorized that this should be happening, nobody had ever seen it, until now.”

A combination of the two black holes in 0402+379 would create a burst of gravitational radiation, like the famous bursts recently discovered by the Laser Interferometer Gravitational-Wave Observatory, but scaled up by a factor of a billion. It would be the most powerful gravitational burst in the universe, Romani said. This kind of radiation burst happens to be what he wrote his first-ever paper on when he was an undergraduate.

Very slow dance

This theorized convergence between the black holes of 0402+379, however, may never occur. Given how slowly the pair is orbiting, the scientists think the black holes are too far apart to come together within the estimated remaining age of the universe, unless there is an added source of friction. By studying what makes this stalled pair unique, the scientists said they may be able to better understand the conditions under which black holes normally merge.

Romani hopes this work could be just the beginning of heightening interest in unusual black-hole systems.

“My personal hope is that this discovery inspires people to go out and find other systems that are even closer together and, hence, maybe do their motion on a more human timescale,” Romani said. “I would sure be happy if we could find a system that completed orbit within a few decades so you could really see the details of the black holes’ trajectories.”

Additional co-authors on this paper are A.B. Peck, Gemini Observatory (formerly of the National Radio Astronomy Observatory); and R.T. Zavala, U.S. Naval Observatory.

This work was funded by NASA and the National Radio Astronomical Observatory.